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I purchased HTC One X unlocked (with AT&T) with Android ICS. I am using this phone in India with Airtel SIM.
How can I upgrade the device to Jelly Bean?
If you have bought an unlocked phone then there's no issue of what kind of carrier you have and which country you are in. Just connect to the internet and get an OTA update from settings, provided they have rolled out the update to their customers in the country where you bought it from.